In these days when the industrial technology field is highly fractionalized, a material used in each technology field has been required to deliver special and high performance. For example, for a steel sheet that is press-formed and put in use, more excellent formability has been required with the diversification of press shapes. In addition, as a high strength has been required, the use of a high-strength steel sheet has been studied. In particular, concerning an automotive steel sheet, in order to reduce the vehicle body weight and thereby to improve the fuel economy from the perspective of global environments, a demand for a high-strength steel sheet having thin-wall high formability has been increasing remarkably. In press forming, as the thickness of steel sheet used is smaller, cracks and wrinkles are liable to occur. Therefore, a steel sheet further excellent in ductility and stretch flangeability is required. However, the press formability and the high strengthening of steel sheet are characteristics contrary to each other, and therefore it is difficult to satisfy these characteristics at the same time.
As a method for improving the press formability of a high-strength cold-rolled steel sheet, many techniques concerning grain refinement of micro-structure have been proposed. For example, Patent Document 1 discloses a method for producing a very fine grain high-strength hot-rolled steel sheet that is subjected to rolling at a total reduction of 80% or higher in a temperature range in the vicinity of Ar3 point in the hot-rolling process. Patent Document 2 discloses a method for producing an ultrafine ferritic steel that is subjected to continuous rolling at a reduction of 40% or higher in the hot-rolling process.
By these techniques, the balance between strength and ductility of hot-rolled steel sheet is improved. However, the above-described Patent Documents do not at all describe a method for making a fine-grain cold-rolled steel sheet to improve the press formability. According to the study conducted by the present inventors, if cold rolling and annealing are performed on the fine-grain hot-rolled steel sheet obtained by high reduction rolling being a base metal, the crystal grains are liable to be coarsened, and it is difficult to obtain a cold-rolled steel sheet excellent in press formability. In particular, in the manufacturing of a composite-structure cold-rolled steel sheet containing a low-temperature transformation product or retained austenite in the metallurgical structure, which must be annealed in the high-temperature range of Ac1 point or higher, the coarsening of crystal grains at the time of annealing is remarkable, and the advantage of composite-structure cold-rolled steel sheet that the ductility is excellent cannot be enjoyed.
Patent Document 3 discloses a method for producing a hot-rolled steel sheet having ultrafine grains, in which method, rolling reduction in the dynamic recrystallization region is performed with a rolling reduction pass of five or more stands. However, the lowering of temperature at the hot-rolling time must be decreased extremely, and it is difficult to carry out this method in a general hot-rolling equipment. Also, although Patent Document 3 describes an example in which cold rolling and annealing are performed after hot rolling, the balance between tensile strength and hole expandability is poor, and the press formability is insufficient.
Concerning the cold-rolled steel sheet having a fine structure, Patent Document 4 discloses an automotive high-strength cold-rolled steel sheet excellent in collision safety and formability, in which retained austenite having an average crystal grain size of 5 μm or smaller is dispersed in ferrite having an average crystal grain size of 10 μm or smaller. The steel sheet containing retained austenite in the metallurgical structure exhibits a large elongation due to transformation induced plasticity (TRIP) produced by the martensitizing of austenite during working; however, the hole expandability is impaired by the formation of hard martensite. For the cold-rolled steel sheet disclosed in Patent Document 4, it is supposed that the ductility and hole expandability are improved by making ferrite and retained austenite fine. However, the hole expanding ratio is at most 1.5, and it is difficult to say that sufficient press formability is provided. Also, to enhance the work hardening coefficient and to improve the collision safety, it is necessary to make the main phase a soft ferrite phase, and it is difficult to obtain a high tensile strength.
Patent Document 5 discloses a high-strength steel sheet excellent in elongation and stretch flangeability, in which the second phase consisting of retained austenite and/or martensite is dispersed finely within the crystal grains. However, to make the second phase fine to a nano size and to disperse it within the crystal grains, it is necessary to contain expensive elements such as Cu and Ni in large amounts and to perform solution treatment at a high temperature for a long period of time, so that the rise in production cost and the decrease in productivity are remarkable.
Patent Document 6 discloses a high-strength hot-dip galvanized steel sheet excellent in ductility, stretch flangeability, and fatigue resistance property, in which retained austenite and low-temperature transformation product are dispersed in ferrite having an average crystal grain size of 10 μm or smaller and in tempered martensite. The tempered martensite is a phase that is effective in improving the stretch flangeability and fatigue resistance property, and it is supposed that if grain refinement of tempered martensite is performed, these properties are further improved. However, in order to obtain a metallurgical structure containing tempered martensite and retained austenite, primary annealing for forming martensite and secondary annealing for tempering martensite and further for obtaining retained austenite are necessary, so that the productivity is impaired significantly.
Patent Document 7 discloses a method for producing a cold-rolled steel sheet in which retained austenite is dispersed in fine ferrite, in which method, the steel sheet is cooled rapidly to a temperature of 720° C. or lower immediately after being hot-rolled, and is held in a temperature range of 600 to 720° C. for 2 seconds or longer, and the obtained hot-rolled steel sheet is subjected to cold rolling and annealing.